The code I used to solve the Time Conversion algorithm on HackerRank

timeFormat.js

function timeConversion(s) {
    // Take the time passed in and check for AM | PM
    // If the time is AM, then just remove the AM unless it's 12
    if (s.endsWith("AM")) {
           // add the first 2 numbers from s to firstNumSet
        let firstNumSet = parseInt(s.substring(0,2));
        
        // if first number is 12, make it 00
        if (firstNumSet === 12) {
            firstNumSet = "00";
        }
        // if number is less than 10 add a zero in front
        else if (firstNumSet < 10) {
            firstNumSet = 0 + firstNumSet.toString();
        }
        
        // take the time and just remove AM from end
        let answer = s.replace("AM", "");
        
        // remove the first set of numbers
        answer = answer.substring(2,8);
        
        // add back on the first number
        answer = firstNumSet + answer;
        
        return answer;
    }
    else {
        // add the first 2 numbers from s to firstNumSet
        let firstNumSet = parseInt(s.substring(0,2));
        
        // unless first number is 12, add 12 to it
        if (firstNumSet !== 12) {
            firstNumSet += 12;
        }
        
        // remove the PM from end
        let answer = s.replace("PM", "");
        
        // remove the first set of numbers
        answer = answer.substring(2,8);
                
        // then add time back to front of String
        answer = firstNumSet + answer;
        // and return the final value
        return answer;
    }

}